The present invention relates to a fixing device applicable to an electronic or a silver halide sensitive type of image forming apparatus.
Fixing systems available in the image forming art may generally be classified into three types, i.e., a heat type system, a pressure type system, and a solvent type system. Among them, the heat type fixing system is practicable with an electrophotographic image forming apparatus. A fixing device implemented with the heat scheme has a heat roller and a press roller facing each other via a sheet transport path and each being rotatably mounted on a respective shaft. The heat roller and press roller are pressed against each other by a spring or similar biasing means. The heat roller melts a toner transferred to a sheet by heating it, while pressing the sheet in cooperation with the press roller during transport. This kind of fixing device is small in size and light in weight since the heat roller heats the sheet and transports it at the same time.
It has been customary with the above-described type of fixing device to configure the heat roller as a hollow cylindrical metallic roller, and a heating resistor body axially extending in the metallic roller. However, the thermal efficiency available with such a heat roller is too low to promote power saving and response. To eliminate this problem, there has been proposed a fixing device having a heat roller whose surface is constituted by a heating resistor, i.e., a surface heat type fixing device. Since this type of fixing device directly causes the surface of the heat roller to generate heat, thermal efficiency is high enough to enhance power saving and response. For example, Japanese Patent Laid-Open Publication No. 164863/1980 teaches a surface heat type fixing device having a rotatable heat roller positioned on a transport path and provided with a plurality of resistors thereon which are separate along the circumference of the roller. In this configuration, only the resistors pressed against a sheet being transported via the surface of the heat roller are driven so as to reduce warm-up time and power consumption.
However, since the conventional surface heat type fixing device heats the entire heat roller, it still needs a long warm-up time and cannot be reduced in size or thickness.
In light of the above, an endless fixing belt and a transport belt may each be passed over a drive roller and a driven roller and be located to face each other via a sheet transport path, as disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 282576/1991 and 282577/1991. In this configuration, a heating member is positioned to face the sheet transport path via the fixing belt. The fixing belt and transport belt convey a sheet in cooperation while the heating member heats the sheet, thereby fixing a toner image carried on the sheet. This kind of scheme reduces warm-up time and power consumption since only part of the heating member pressed against the sheet via the fixing belt has to generate heat, i.e., it is not necessary for the entire fixing belt or heating member to generate heat. The fixing belt is made up of a heat-resisting layer and a separating layer formed on the heating-resisting layer in order to be resistive to heat and separable from a mold. Specifically, the heat-resisting layer is made of polyimide, polyether ketone, polyether sulfone, polyether imide, polyparabanic acid or similar resin, or nickel, aluminum or similar metal. On the other hand, the separating layer can be formed of polytetrafluoroethyrene or a similar fluoric resin, or silicon resin.
The conventional fixing device, whether it be implemented with a surface heating roller or a fixing belt, heats the entire surface of a sheet, as stated above. This is wasteful in respect of power consumption, since the heating member is wastefully driven despite the fact that a toner image to be heated occupies only a small part of the sheet. Particularly, the fixing device using a fixing belt causes the heating member thereof to heat the sheet by way of the belt, resulting in low thermal efficiency and substantial power consumption. While the thickness, among others, of the fixing belt may be reduced to enhance thermal efficiency, then the durability of the belt will be reduced. Further, although the fixing belt may be implemented by a metallic film having high thermal conductivity, such a belt will sequentially conduct the heat from the heating member along the surface thereof, again resulting in low thermal efficiency.